linux-kernel - [PATCH v4 3/6] KVM: x86: Ignore CR0.WP toggles in non-paging mode

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Date:   Wed, 22 Mar 2023 02:37:28 +0100
From:   Mathias Krause <minipli@...ecurity.net>
To:     kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mathias Krause <minipli@...ecurity.net>
Subject: [PATCH v4 3/6] KVM: x86: Ignore CR0.WP toggles in non-paging mode

If paging is disabled, there are no permission bits to emulate.
Micro-optimize this case to avoid unnecessary work.

Suggested-and-co-developed-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Mathias Krause <minipli@...ecurity.net>
---
 arch/x86/kvm/x86.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c6d909778b2c..8a66ac7a4878 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -908,14 +908,20 @@ void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned lon
 {
 	/*
 	 * CR0.WP is incorporated into the MMU role, but only for non-nested,
-	 * indirect shadow MMUs.  If TDP is enabled, the MMU's metadata needs
-	 * to be updated, e.g. so that emulating guest translations does the
-	 * right thing, but there's no need to unload the root as CR0.WP
-	 * doesn't affect SPTEs.
+	 * indirect shadow MMUs.  If paging is disabled, no updates are needed
+	 * as there are no permission bits to emulate.  If TDP is enabled, the
+	 * MMU's metadata needs to be updated, e.g. so that emulating guest
+	 * translations does the right thing, but there's no need to unload the
+	 * root as CR0.WP doesn't affect SPTEs.
 	 */
-	if (tdp_enabled && (cr0 ^ old_cr0) == X86_CR0_WP) {
-		kvm_init_mmu(vcpu);
-		return;
+	if ((cr0 ^ old_cr0) == X86_CR0_WP) {
+		if (!(cr0 & X86_CR0_PG))
+			return;
+
+		if (tdp_enabled) {
+			kvm_init_mmu(vcpu);
+			return;
+		}
 	}
 
 	if ((cr0 ^ old_cr0) & X86_CR0_PG) {
-- 
2.39.2